Method of centrifugally casting metal under an inert atmosphere

ABSTRACT

This invention relates to a method of casting metal in a rotary mould or die under the protection of an inertizing atmosphere formed by a liquefied gas. In the mould or die, successive casting operations are performed with different metals while maintaining an infeed of liquefied gas during each of these casting operations and while the metal involved is solidifying, until an end product is obtained which is formed from a plurality of layers of different metals. The invention is applicable in particular, but not exclusively, to the production of centrifugally cast tubes and rolling-mill rolls.

BACKGROUND OF THE INVENTION

The present invention relates to methods of casting metal in a rotarymould or die, of the kind in which protection is provided simultaneouslyfor the surface of the bath of metal and the space within the mould,prior to casting, by means of a controlled stream of liquefied inert gaswhich on the one hand is directed over the surface of the bath and whichon the other hand is fed into the interior of the mould as it is drivenin rotation. Hereinafter, such a method will be referred to as "of thekind described".

This method is applicable to the manufacture of centrifugally casttubes. Before the metal is introduced into the mould or die, there isfirst introduced into it a sufficient quantity of liquefied inert gas togive an inert period which will allow the time required to perform thecasting proper. With this manufacturing technique, which presupposesthat metal is cast only once, the liquefied gas is only introduced priorto the casting.

It is an object of the invention to enable parts formed from a pluralityof metals, which thus require a plurality of successive castingoperations in the rotary die, to be manufactured.

There are many metal parts which being bodies of revolution, areproduced by moulding in a rotary mould or die and which, to withstandthe conditions under which they are forced to operate, need to exhibitcertain properties to a vary pronounced degree. Thus, ducts intended forconveying corrosive liquids and pipes intended to be submerged in thesea need to exhibit a high resistance to corrosion, whereas rolls forrolling mills need to have circumferential surfaces of extreme hardness.However, metals or alloys which have the requisite qualities areextremely costly and this results in their being used to form only apart of the final article to be obtained, the remainder of the saidarticle being formed from an ordinary, less costly metal. In the case ofa duct intended to convey a corrosive liquid for example, only theinside wall which is in direct contact with the said liquids needs toexhibit high resistance to corrosion, to a depth which may be small, andthe remainder of the wall, which simply provides mechanical strength,may be formed from ordinary cast iron or steel. Similarly, in the caseof a rolling-mill roll only the body, that is to say the outer workingsurface, needs to be extremely hard, while there is no need for the coreof the roll to be of extreme hardness and indeed it is preferable for itto exhibit other characteristics, such as a certain amount ofresilience, which leads to its being produced from a relativelymalleable and inexpensive material such as grey cast iron.

To produce such articles it is necessary to cast a plurality of metals,generally two, of different kinds in succession in the rotary mould ordie, and thus to perform the moulding in a plurality of stages. Thefirst casting operation allows a first layer to be obtained and a secondcasting operation, which is performed after the time required for thefirst layer to solidify, allows a second layer to be obtained, and soon.

This procedure has many drawbacks. The mould or die, which is driven inrotation and which is normally preheated, forms a hot, open enclosurewhich sucks in the surrounding air vigorously. The result is that thecasting and solidification of the layers of metal take place in anenclosure through which a stream of very hot and turbulent air flows.When a layer has solidified under these conditions it becomes oxidisedon its inner face and this prevents the layer of metal added by the nextcasting operation from adhering properly. The bond between the twolayers is seriously defective and this detracts from the quality of thefinished article.

It is therefore a further object of the invention to overcome orminimise these disadvantages and to enable this object to be achieved.

SUMMARY OF THE INVENTION

Accordingly, in a method of the kind described the invention proposesthe following steps:

(a) a first casting operation is prepared with a first metal which isapplied, by centrifuging, against the wall of the mould or die and whichsolidifies to form a first outer layer of said first metal, inertliquefied gas continuing to be introduced into the mould or die duringthis first casting operation and during the solidification, the gas thusfilling the cavity inside said first layer, and then, after said firstlayer has solidified,

(b) a second casting operation is performed with a second metal which isapplied, by centrifuging, against the inside wall of said first layer soas to fill at least part of said cavity inside said first layer and soas to form a second layer internal to the first layer, the infeed ofliquefied gas continuing during said second casting operation and ifdesired while the metal is solidifying, the gas filling the cavityinside the said second layer at least until it has solidified, and so onuntil a body-of-revolution end product is obtained which is formed fromtwo or more successive layers of different metals.

The protection from the corrosive action of the air which is obtained inthis way during the casting and solidification phase enables solidifiedlayers of metal to be obtained which are completely free from surfaceoxidation, thus enabling an excellent bond between two successive layersto be obtained.

In accordance with another feature of the invention, a plurality oflayers is formed while leaving a hollow space inside the final layer,thus producing a tubular end product, the liquefied gas filling thehollow space in the course of the solidification of the last layer.

In this way ducts or pipes can be obtained in which the outer part ofthe final layer, which is cast last, that is to say on the inside of thefinished pipe or duct, is free from any oxidation.

In accordance with another feature of the invention, a plurality ofcasting operations is performed, the final casting operation filling upthe whole of the space enclosed by the layer formed by the castingoperation which preceded it, thus producing a solid end product.

This method of casting is used to produce rolling-mill rolls.

The liquefied gas may be introduced into the mould or die during thesuccessive casting operations and in the course of the solidificationphase, in a quantity per minute which corresponds to a volume of gas inthe gaseous phase equal to five or ten times the volume of the cavity tobe filled, thus maintaining in the cavity an atmosphere having an oxygencontent of less than 0.1%.

Such an oxygen content enables oxidation to be avoided or minimised.

BRIEF DESCRIPTION OF THE DRAWINGS

Other advantages and features of the invention will become apparent froma purusal of the following description, taken in conjunction with theaccompanying drawings, which are given solely by way of example, and inwhich:

FIG. 1 is a schematic view of the production, by centrifuging, of atube,

FIG. 2 shows the production of a rolling-mill roll, and

FIG. 3 is a cross-section view of a tube made up of two successivelayers which has undergone a crushing test.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The production of body-of-revolution articles formed from a plurality ofmetals or alloys has to be performed in a plurality of stages in rotarymoulds or dies which may have either horizontal or vertical axes, thefirst being used chiefly for producing hollow articles, the second forproducing solid articles.

Referring now to the drawings, the mould or die 1 in FIG. 1 is a steeldie of tubular shape having a horizontal axis. It is supported by tworollers 2 which drive it in rotation about its axis XX' and its two endsare closed off by two end-pieces 3a, 3b which are provided withrespective openings 4a, 4b. The molten metal M, which is contained in aladle 5, flows out into a pouring basin 6 and pours into the interior ofthe mould or die 1 through a spout 6a which passes through the opening4b in end-piece 3b. The metal in the ladle 5 and in the basin 6, andalso the stream of metal dropping into the basin are protected by thecontinuous application of an inertising liquefied gas by means ofdevices of a known type such as nozzles 7a and a ring 7b. The liquefiedgas exemplified as nitrogen, is introduced into the interior of themould 1 by a nozzle 8 which passes through the opening 4a in end-piece 3a.

The aim is to use this mould or die to produce a tube formed from twolayers of metals of different kind (a bimetallic tube). The tube has alength of approximately 4 meters, an outside diameter of 30 cm, aninside diameter of 25 cm and thus a thickness of 2.5 cm.

The casting takes place as follows:

The die, which is driven in rotation by any suitable means well-known inthe art, but not illustrated, is first purged and inertised. Theliquefied nitrogen, which is poured in at a constant rate of two litersper minute for three minutes, quickly fills the die and, by virtue ofthe rotation, spreads uniformly over the entire length of its insidewall. As soon as the liquefied gas enters, the air contained in the dieis forcibly expelled since the gaseous expansion as a result ofevaporation is very great, one liter of liquid nitrogen at 15° C.producing approximately 680 liters of gas. In this way a nitrogenatmosphere containing less than 0.1% oxygen is obtained within themould.

A first metal or alloy, such as nickel-chrome, is then cast, to producean outer layer 9 approximately 5 mm thick. During the casting of thisfirst metal, which lasts approximately one minute, liquid nitrogencontinues to be poured in at the abovementioned rate.

There is then a wait of approximately three minutes until the firstlayer 9 has solidified, the infeed of nitrogen remaining constant duringthe solidification.

A second metal or alloy different from the first, such as cast iron forexample, is then cast to produce a second layer 10 approximately 20 mmthick. During the second casting operation, which lasts approximatelyone minute, liquid nitrogen continues to be fed in at the abovementioned rate. The die is allowed to rotate and the infeed of liquidnitrogen is continued for approximately one minute to ensure that thesecond layer 10 has solidified sufficiently.

The rotation of the die is then slowed down and the finished tube isextracted.

The total length of the operation is approximately nine minutes and thequantity of liquified nitrogen is approximately eighteen liters.

The final tube obtained thus has a thin outer layer 9 having for examplehigh resistance to corrosion, and an inner layer 10 which is muchthicker and which is made for example from a far less expensive metalthan the outer layer.

The fact of first having purged the die and then having maintained anoxygen-free atmosphere during the casting of the first metal and duringits solidification and also during the casting of the second metal untilit has solidified makes it possible to prevent any pollution of theinner face of the first layer and thus ensures an excellent bond betweenthe first layer and the second layer. The absence of pollution is duenot only to the absence of oxygen but also to the absence of watervapour, which is always present in atmospheric air.

The mould or die 12 in FIG. 12 is a steel die of cylindrical shapehaving a vertical axis, whose diameter is approximately 0.35 meters andwhose height is approximately 0.70 meters. At the bottom, the mould ordie 12 continues into a mould or die 13, which is also of cylindricalshape but of substantially smaller dimensions, the second die having adiameter of approximately 0.15 meters and a height of approximately 0.20meters. The assembly formed by the dies 12 and 13 is driven in rotationabout its vertical axis YY' at approximately 800 rpm by any convenientmeans which are not shown, but which are well-known in the art. At thetop, the die 12 is closed off by a cover 14. The cover 14 is providedwith an opening 15 through which passes a casting spout 16 which isconnected to a pouring basin (not shown) and which is surrounded by ajacket 17. The inert liquefied gas is fed in through a heat-insulatednozzle 18 which is connected to a source (not shown). The liquefied gasthus enters the die through the opening 15 parallel to the flow of metalentering through the spout 6.

In this die, the aim is to produce a roll for a rolling-mill formed byan outer layer of high mechanical strength and an inner core made from aless strong but more malleable metal.

The sequence of operations is closely comparable with that employed inthe production of the centrifugally cast tube.

The mould or die, which is driven in rotation, is first of all purgedand inertised by feeding into it liquefied gas, which is nitrogen inthis case, at a rate of two liters per minute for approximately 11/2minutes.

The first metal, such as nickel cast iron, is then cast to produce afirst outer layer 19, or body, having a thickness of approximately 1 cm.During this casting operation, which lasts approximately thirty seconds,the die continues to be fed with liquid nitrogen.

While the die continues to be fed with liquid nitrogen, there is a waitof approximately six minutes for the body to solidify.

A second metal, such as grey cast iron, is then cast and this metalfills the interior of the die 12 to produce a core 20, and the interiorof the die 13 to produce a journal 21. During the casting of this secondmetal, which lasts approximately one minute, the liquefied nitrogencontinues to be supplied until the end of the casting operation.

The mould is then slowed down and stopped and the roll extracted.

The total length of the operation is approximately nine minutes and theconsumption of liquefied nitrogen is approximately eighteen liters.

In this way there is obtained a rolling-mill roll having a weight ofapproximately 500 kg. and a diameter of approximately 0.35 meters. Theroll is formed by an outer body of extreme hardness and high abrasionresistance, and an inner core which is of greater resilience and has agreater capacity for flexing.

As in the previous case, the interface between the two layers issubstantially free of any pollution or oxidisation and there is thusexcellent cohesion between the two layers.

Examination of articles produced by this method shows that the adhesionbetween the two layers remains excellent even after crushing tests.

FIG. 3 is a cross-sectional view of a tube obtained by the mouldingmethod illustrated in FIG. 1 and shows that when such a tube is crushedafter having been split longitudinally there is no tendency for thelayers 9 and 10 to separate even in the region A of maximum deformation.

It would of course be possible to make many modifications to the twomethods described and illustrated above. Thus, it would be possible tohave articles which, instead of two layers, were formed from threelayers or more. In this case the die would be supplied with liquefiedgas during the casting of each layer and during its solidification. Theliquefied gas could be formed by any gas other than nitrogen, such asargon.

The method is equally applicable to all plain metals or metalliccompounds or alloys. Examples of suitable plain metals are iron, copperand chromium. Examples of suitable metallic compounds are cast iron andsteel. Examples of suitable alloys are nickel-chrome and nickel-castiron. Other plain metals, metallic compounds and alloys may of course beemployed as will now be apparent to these skilled in the art to whichthis invention relates.

The thickness of the layers may also be as desired, the thicknessdepending solely on the properties demanded from the article finallyobtained.

We claim:
 1. In a method of casting metal in a rotary mold, the stepsof:continuously rotating the mold; introducing inert liquified gas intothe rotating mold to spread the liquid over the inner surface of saidmold; directing a first molten metal into the rotating mold, the firstmetal being urged by centrifuging against the inner surface of the moldand solidifying therein to form a first metallic layer; continuing theintroduction of inert liquified gas into the rotating mold as the firstmolten metal is directed thereto and during the solidification of thefirst metal, the gas filling the cavity formed by said first metalliclayer; said inert liquified gas being introduced into said mold duringthe insertion of said first molten metal at a rate corresponding to avolume of gas in the gaseous phase equal to at least five times thevolume of the cavity to be filled, to maintain in the cavity anatmosphere having an oxygen content of less than 0.1%; and directing asecond molten metal into the rotating mold after the solidification ofthe first metal, the second metal being urged by centrifuging againstthe inner surface of the cavity formed by said first layer andsolidifying therein to form a second metallic layer.
 2. In a method ofcasting metal in a rotary mold, the steps of:continuously rotating themold; introducing inert liquified gas into the rotating mold to spreadthe liquid over the inner surface of the mold; directing a first moltenmetal into the rotating mold, the first metal being urged bycentrifuging against the inner surface of the mold and solidifyingtherein to form a first metallic layer; continuing the introduction ofinert liquified gas into the rotating mold as the first molten metal isdirected thereto and during the solidification of the first metal, thegas filling the cavity formed by said first metallic layer; directing asecond molten metal into the rotating mold after the solidification ofthe first metal, the second metal being urged by centrifuging againstthe inner surface of the cavity formed by said first layer andsolidifying therein to form a second metallic layer; and continuing theintroduction of inert liquified gas into the rotating mold as the secondmolten metal is directed thereto; said inert liquified gas beingintroduced into said mold during the insertion of at least one of saidmolten metals therein at a rate corresponding to a volume of gas in thegaseous phase equal to at least five times the volume of the cavity tobe filled, to maintain in the cavity an atmosphere having an oxygencontent of less than 0.1%.
 3. In a method of casting metal in a rotarymold, the steps of:continuously rotating the mold at a constant speed;introducing inert liquified gas into the rotating mold to spread theliquid over the inner surface of said mold; directing a first moltenmetal into the rotating mold, the first metal being urged bycentrifuging against the inner surface of the mold and solidifyingtherein to form a first metallic layer; continuing the introduction ofinert liquified gas into the rotating mold as the first molten metal isdirected thereto and during the solidification of the first metal, thegas filling the cavity formed by said first metallic layer; directing asecond molten metal into the rotating mold after the solidification ofthe first metal, the second metal being urged by centrifuging againstthe inner surface of the cavity formed by said first layer andsolidifying therein to form a second metallic layer; and continuing theintroduction of inert liquified gas into the rotating mold as the secondmolten metal is directed thereto and during the solidification of thesecond metal; said inert liquified gas being introduced into said moldduring the insertion of at least one of said molten metals therein at arate corresponding to a volume of gas in the gaseous phase equal to atleast five times the volume of the cavity to be filled, to maintain inthe cavity an atmosphere having an oxygen content of less than 0.1%. 4.In a method of casting metal in a rotary mould or die, in whichprotection is provided simultaneously for the surface of the bath ofmetal and for the space within said mould or die, prior to casting, bymeans of a controlled stream of inert liquified gas which on the onehand is directed over the surface of the bath and on the other hand intothe interior of said mould or die, which is driven in rotation, theinvention which comprises the following steps:(a) a first castingoperation is performed with a first metal which is applied, bycentrifuging, against the wall of said mould or die and which solidifiesto form a first outer layer of said first metal, said inert liquifiedgas continuing to be introduced into said mould or die during this firstcasting operation and during the solidification, the gas thus fillingthe cavity inside said first layer, and then, after said first layer hassolidified, (b) a second casting operation is performed with a secondmetal which is applied, by centrifuging, against the inner wall of saidfirst layer so as to fill at least part of the space inside said firstlayer and so as to form a second layer internal to said first layer, theinfeed of said liquified gas continuing during said second castingoperation, said gas thus filling the cavity inside said second layer atleast until it has solidified, (c) said steps are repeatedly carried outuntil a body-of-revolution end product is obtained which is formed fromat least two successive layers of different metals, and (d) saidliquified gas being introduced into said mould or die during thesuccessive casting operations and in the course of the solidificationphase in a quantity per minute which corresponds to a volume of gas inthe gaseous phase equal to five to ten times the volume of the cavity tobe filled, to maintain in the cavity an atmosphere having an oxygencontent of less than 0.1%.